1. Field of the Invention
The present invention relates to a radio for a W-CDMA (Wideband Code Division Multiple Access) mobile communication system and the like, which has a distortion compensation capability to reduce Adjacent Channel Leakage power by compensating a distortion of a transmission signal and to achieve high power-added efficiency of a power amplifier.
2. Description of the Related Art
I. Distortion and Efficiency of the High Power Amplifier.
In wireless communication systems, especially in mobile communication systems such as W-CDMA and PDC, the transmit power is high and ranges from 10 mW to several tens of Watts. Since the transmit power has to be controlled according to the distance between one communication device and the other, the dynamic range of the transmit power becomes significantly wide. Thus, it is necessary to make use of a high power amplifier (HPA) that can avoid distorting a signal even at maximum transmit power.
FIG. 1 illustrates plots of output power and power-added efficiency versus input power of the high power amplifier (HPA). As illustrated, while the power-added efficiency of the HPA is high in a non-linear area of the transmit power (output power of the HPA), it is low in a linear area in which the transmit power (output power of the HPA) is low and the requirement as to an Adjacent Channel Leakage power Ratio (ACLR) is met.
The term “power-added efficiency” used herein expresses the power ratio, which is derived by dividing the power added by the HPA (the power difference between the output power and the input power) by the product of the power voltage and the supply current applied to the HPA (the power consumption of the HPA).
The leakage power into an adjacent channel is strictly regulated, because it constitutes noise in other channels and causes degradation of the communication quality of the other channels and produces a detrimental effect on the capacity of the communication, as shown in FIG. 2A. Here, the Adjacent Channel Leakage power Ratio (ACLR) is an index for regulating the leakage power, which has also been referred to as Adjacent Channel Power Ratio (ACPR).
As for a detailed explanation, the first Adjacent Channel Leakage power Ratio (ACLR1) is the ratio between the output power P1 of the channel indicated by the diagonally hatched area shown in FIG. 2B and the output power (PL1 or PH1) of one of the first adjacent channels indicated by the vertically hatched area shown in FIG. 2B, as follows.ACLR1=(PL1 or PH1)/P1  (1)
Likewise, the second Adjacent Channel Leakage power Ratio (ACLR2) is the ratio between the output power P1 of the channel (diagonally crosshatched) and the output power (PL2 or PH2) of one of the second adjacent channels indicated by the horizontally hatched area shown in FIG. 2B, as follows.ACLR2=(PL2 or PH2)/P1  (2)
FIG. 3A shows the non-linear area and the linear area of the HPA. While the leakage power into an adjacent channel is low in the linear area in which the output power of the HPA is proportional to the input power, it is high in the non-linear area in which the output power of the HPA is not proportional to the input power.
Therefore, it may be possible to use the HPA whose linear area is wide (i.e., the HPA whose maximum output power that cannot cause a distortion is high) for the purpose of the high output power and the suppression of leakage power. However, the HPA has to be equipped with capability greater than is required in its practical use, which can lead to an increase in cost and size.
On the other hand, with the traditional HPA, the power-added efficiency is significantly lower in the linear area. Thus, when the transmission is accomplished at a certain predetermined output power, the wasted consumption power of the traditional HPA ranges from several times to several tenths of the output power. In other words, it has a disadvantage in terms of increased consumption power.
II. Control of a Bias Voltage of the HPA.
Under the circumstances, the techniques for controlling the bias voltage of the HPA so as to reduce the consumption power have been proposed (for example, Japanese laid-open patent publication 2001-019792).
FIG. 3B shows the graph, in which the power-added efficiency is plotted versus the transmit power (the output power of the amplifier) which meets the specification as to the Adjacent Channel Leakage power Ratio (ACLR). In this figure, the plots of the power-added efficiency at bias voltage A1[V] are indicated by the symbol ◯, and the plots at bias voltage A2[V] are indicated by the symbol Δ and, the plots at bias voltage A3[V] are indicated by the symbol X, where A3<A2<A1. As illustrated, the power-added efficiency increases as the bias voltage decreases.
As for a detailed explanation, as shown in FIG. 4A, it is given that the power-added efficiency is η3 when the bias voltage A1[V] is applied to the HPA with the transmit power Pa. Then, the power-added efficiency at the bias voltage A2[V] (A2<A1) is η2 (η2>η3) and the power-added efficiency at the bias voltage A3[V] (A3<A2) is η1 (η1>η2) with the transmit power Pa.
FIG. 4B shows the effects of controlling the bias voltage, in which a dashed line indicates the graph without bias voltage control and a solid line indicates the graph with bias voltage control. In this way, by appropriately controlling the bias voltage of the HPA so that the Adjacent Channel Leakage power Ratio (ACLR) is met and the power-added efficiency is improved, it becomes possible to improve the power-added efficiency versus the transmit power (the output power of HPA). However, in the case of high output power, the power-added efficiency without bias voltage control is better than the power-added efficiency with bias voltage control.
FIG. 5 shows a block diagram for controlling the bias voltage of HPA 21-4 according to prior art. In this embodiment according to prior art, first of all, transmit power is determined at a bias voltage controller 21-1 based on a Transmit Power Control (TPC) signal. Then, the bias voltage (power supply voltage) is determined appropriately such that the power-added efficiency becomes the maximum.
Then, based on the result of this determination, the bias voltage controller 21-1 sets the output power of DC—DC converters 21-2, 21-3. Although the input bias and the output bias of the HPA 21-4 are controllable by bias tees 21-5 21-6, respectively, in this arrangement shown in FIG. 5, an arrangement in which only one of the input bias and the output bias is controllable may be applicable, as far as the power-added efficiency is improved.
III. Distortion Compensation.
As has been discussed, to reduce the consumption power and keep the power-added efficiency high, the use of the non-linear area of the HPA is indispensable. However, the use of the non-linear area causes an increase of the distortion level, which can lead to the degradation of the Adjacent Channel Leakage power Ratio (ACLR) shown in FIG. 2.
Under these contradictory conditions, a distortion compensator (linearizer) is proposed as a means of keeping the transmit power and the power-added efficiency high while suppressing the distortion. The linearizer compensates the distortion of the waveform of the transmission signal at the high transmit power so as to keep the power-added efficiency high. The detailed information about the linearizer is disclosed in JP 09-69733A and JP 2000-251148A, etc.
FIG. 6A shows the fundamental block diagram of the linearizer. The linearizer is provided with an adaptive distortion compensation controller 22-10 that generates a distortion compensation coefficient according to the amplitude of the baseband signal x (t) using an adaptive algorithm.
The amplitude distortion and the phase distortion f (p) of the HPA 22-30 with respect to a certain power level of the baseband signal x (t) is expressed as a complex number. The amplitude distortion and the phase distortion f (p) of the HPA 22-30 are compensated at a multiplier 22-20 as a result of multiplying the baseband signal x (t) by the distortion compensation coefficient of a complex number determined according to the amplitude so as to pre-distort the baseband signal x (t).
The adaptive distortion compensation controller 22-10 comprises a distortion compensation table 22-11 for storing and retaining the distortion compensation coefficients and a distortion compensation coefficient generator 22-12. The distortion compensation table 22-11 stores and reads out the distortion compensation coefficient according to the amplitude.
The distortion compensation coefficient generator 22-12 estimates the distortion compensation coefficient based on a difference signal e (t) output from a subtracter 22-40 that calculates the difference between the baseband signal x (t) and the output signal from the HPA 22-30. The estimated distortion compensation coefficient is stored and retained in the distortion compensation table 22-11. An adaptive algorithm is made use of for this estimation. There are various types of adaptive algorithms commonly known. For detailed information about these algorithms, reference should be made to a book such as “An Introduction to Adaptive Filters” by S. Haykin.
FIG. 6B shows the diagram of a circuit for estimating and generating the distortion compensation coefficients. As shown in FIG. 6B, it comprises a distortion compensation table 22-11 for storing and retaining the distortion compensation coefficients, an adder 22-13 for computing the distortion compensation coefficients based on the difference signal e (t), multipliers 22-14, 22-15, 22-16, and a complex conjugate transform circuit 22-17.
The distortion compensation coefficient h (p) is calculated as follows.hn(p)=hn-1(p)+μe(t)u*(t)  (3)e(t)=x(t)−y(t)  (4)u(t)=x(t)f(p)≈h*n-1(p)y(t)  (5)hn-1(p)h*n-1(p)≈1  (6)y(t)=h*n-1(p)x(t)f(p)  (7)p=|x(t)|2  (8)
Where hn (p) is the distortion compensation coefficient updated n times, hn-1 (p) is the distortion compensation coefficient updated n-1 times (the last time), μ is the step-size parameter as to the update amount, y (t) is the output signal of the HPA, and f (p) is a distortion function for the HPA. Further, x (t), y (t), f (p), hn (p), hn-1 (p), u (t), and e (t) are complex numbers, and the symbol “*” indicates a conjugate complex number. Further, u (t) is a value approximated according to the equation (5) on the assumption that the amplitude distortion of the HPA is not so large (i.e., hn-1(p)h*n-1(p)≈1).
hn (p) of the equation (3) is the distortion compensation coefficient to be updated this time and input to the distortion compensation table 22-11. y* (t) is output from the complex conjugate transform circuit 22-17 that generates the conjugate complex number according to the output signal y (t) of the HPA. Accordingly, the output of the multiplier 22-16 is y* (t) hn-1 (p).
Then, the output of the multiplier 22-16 is multiplied by the output e (t) of the subtracter 22-40. Accordingly, y* (t) hn-1 (p) e (t) is output. Then, after being multiplied by the step-size parameter μ at the multiplier 22-14, it is added to hn-1 (p) at the adder 22-13.
Accordingly, the estimated distortion compensation coefficient hn (p) to be updated is as follows.hn(p)=μy*(t)hn-1(p)e(t)+hn-1(p)  (8)Here, given that u (t)≈h*n-1 (p) y (t) as in the equation (5), u* (t) is as follows.u* (t)=y*(t)hn-1(p)  (10)Consequently, the distortion compensation coefficient hn (p) is expressed as the equation (3).
The equation (8) is the one for calculating the level of the power of the baseband signal and it is calculated at an electric power calculator 22-18. If the equation (8) is the one for calculating the amplitude of the baseband signal x (t), the equation (8) will be expressed as p=|x (t)|. Alternatively, if p is the function of the electric power or the amplitude, p may be calculated as p=g(|x(t)|2) or p=g(|x(t)|).
The value of p calculated at the electric power calculator 22-18 indicates the address for the distortion compensation table 22-11 at the time of writing and reading. If the update operation by this writing and the operation of multiplying the baseband signal by the distortion compensation coefficient are carried out independently, it is always possible to apply the pre-distortion without being subject to the influence of the delay of the system as a whole. Although the example of the adaptive algorithm using a least mean squares method (LMS) is disclosed herein, the use of a clipped LMS algorithm or an exponential weighting RSL algorithm is applicable (for example, see JP 09-69733A).
By the way, in the case of controlling the bias voltage of the HPA according to the prior art as shown in FIG. 5, the specification as to the Adjacent Channel Leakage power Ratio (ACLR) should be met. For this reason, it becomes impossible to accomplish the best power-added efficiency, if the bias voltage of the HPA is set to a fixed value of the power supply voltage. In other words, one can only hope for the improvement as shown in FIG. 4B.
On the other hand, according to the arrangement using the linearizer as shown in FIG. 6, the suppression of the distortion by multiplying the input signal by the distortion compensation coefficient permits use of the non-linear area of the HPA. Accordingly, the improvement of the power-added efficiency can be attained when the transmit power is relatively high. However, the power-added efficiency becomes low when the transmit power is low, as shown in FIG. 1.